Studies on the reactivity of the sulfur-sulfur bond of protein bound cystine residues using synthetic model peptides are in progress. The effect of chemical modification and bound inhibitors on the conformational mobility of a nitroxide-type spin-label located at His-105 in pancreatic ribonuclease A has been studied. BIBLIOGRAPHIC REFERENCES: "Sulfur-Containing Polypeptides. XVII. The S-Carbomethoxysulfenyl Derivative as a Protective Group for Cysteine", J. Org. Chem., 40, 950 (1975); R.G. Hiskey, N. Muthukumaraswamy and R.R. Vunnam. "Synthesis of D,L-gamma-Carboxyglutamic Acid Derivatives", J. Org. Chem., 40 2850 (1975); N.T. Boggs III, R.E. Gawley, K.A. Koehler and R.G. Hiskey.